Specialised valves, including blow out preventers (BOPs), have been available for many decades, and provide a means for sealing the wellbore at the wellhead against well pressure if the drilling crew loses control of formation of fluids or if required to seal a wireline or logging cable during well service operations. A conventional ram type BOP is located at the wellhead, and consists of two halves of a cover for the wellbore, located on diametrically opposed sides of the well bore. The covers consist of steel rams and elastomeric ram blocks, and are typically actuated by large diameter hydraulic cylinders located on each side of the wellbore. If a well control event occurs, the hydraulic cylinders will be remotely actuated to force the rams to enter the wellbore from either side, meeting at the centre to seal against well pressure below. Importantly, the BOP must have the ability to seal the wellbore without cutting the wireline, allowing subsequent retrieval of the toolstring.
The ram blocks will typically have cooperating, shaped surfaces to ensure alignment. The ram blocks are available in a variety of configurations, and for applications where pipeline or wireline is being run in the well will be provided with a cut-out formation (typically a corresponding pair of semi-circular recesses defining a narrow throughbore) designed to allow the wireline or pipeline to extend through the BOP without damage. The ram blocks will be designed such that a tight seal is formed around the pipeline or wireline and around the aperture through which the rams move. The ram blocks will often be provided with a guide, such that when the rams move from an open to a closed position, the pipeline or wireline is guided into the centre of the wellbore such that it is received in the cut-out formation without damage.
The rams in a typical ram BOP are self-energising, in the sense that after they are initially actuated by the hydraulics, well pressure is allowed to get behind a part of the ram, forcing it towards a closed position. The elastomeric seals of the ram blocks are U shaped in longitudinal section through the plane which the rams extend, and extend fully across the opening of the wellbore. The U-shaped seals allow the well pressure to force the seals upward and inward, such that the well pressure energises the seal and tends to keep the rams in a sealed configuration.
Other variations on the ram BOP are available, including rams designed to completely shear through pipe or wireline. In addition, some arrangements are manually operated rather than hydraulically operated. In this case, the actuators normally consist of large screws and are rotated by use of a large handle to generate the torque required to drive the rams against the well pressure.
Presently available ram type BOPs suffer from a number of deficiencies and drawbacks. In particular, many of the drawbacks arise from the size of the apparatus. Available designs consist of a pair of rams arranged perpendicular to the wellbore, and extending on either side of the wellbore by up to 2.5 meters (around 100 inches). This has significant implications for the construction of well control apparatus at the rig. Ideally, the BOP stack would be located close to the wellhead, to maximise the height above available to accommodate the toolstring. However, in practice, it is difficult to manoeuvre the BOP stack through platforms on the rig structure. A platform on the rig structure would typically have a hole in the grating of around 24 inches diameter (depending on the diameter of pipe passing through). This opening is too small to allow a ram-type BOP to be passed through in its usual upright configuration where the hydraulic actuators are oriented horizontally. This requires the BOP to be manoeuvred carefully such that the cross-axis access is oriented vertically, allowing the BOP to be passed through the opening. This is a difficult and dangerous operation due to the size and weight of the BOP stack. A reduction in weight is limited by the need for large diameter hydraulic rams and high strength steel components.
As an alternative, the platform may be provided with a larger diameter slot to allow the passing of the BOP to be conducted more easily. However, this has implications for the health and safety of personnel working at the platform, and may require installation railings or other barriers to prevent personnel from falling through the slots. It is therefore more usual to install the BOP stack at an elevated height above the wellhead, of in excess of 13 meters (40 feet). The consequence is that a greater overall height of well control apparatus is required, or alternatively restrictions are placed on the length of tool string available. This deficiency is particularly important for concurrent operations.
Existing BOP designs suffer from other deficiencies. These include problems with the structural strength of the ram. The ram blocks located on either side of the wellbore will typically include a recess and protrusion which cooperate to ensure that the ram blocks are appropriately aligned. The recess formed on a ram will reduce the structural strength in the ram. Moreover, the recess may in fact be formed on an outer surface of the ram, meaning that as the ram is sealing it has a portion that is unsupported. The ram is therefore “floating” rather than being fully supported by the body of the BOP. This can also result in deformation of the ram, impacting on its application to high pressure BOPs.
The arrangement of a typical ram BOP can also cause difficulties in guiding the pipeline or wireline to the right part of the ram block formation (i.e. the cut-out formation). These difficulties arise in part due to the problems with keying the rams appropriately. In addition, typically the piston assembly on one side of the wellbore will be slightly looser than the opposing ram, resulting in the loose piston assembly being driven to the full extent of its travel before the opposing piston assembly begins to move. This increases the probability of the wireline or pipeline being snagged or damaged by the guiding profile. This can damage the wireline or pipeline and in severe cases cause it to be sheared.
A further deficiency of typical ram-type BOPs is the time taken to actuate the rams, due to the high volume hydraulic cylinders, or worse, the time required to manually close the rams. This has safety implications.
The present applicant has identified the need for an improved valve design, and in particular an improved BOP design that differs significantly from a conventional ram type BOP.
It is a first aim of the invention to provide an apparatus suitable for sealing a bore that obviates or at least mitigates some of the drawbacks and deficiencies of available valves and/or BOPs. In particular, it is an aim of at least one aspect of the invention to provide an improved valve or BOP for sealing around a wireline, pipeline or conduit in a wellbore.
It is a further aim of the invention to provide apparatus for sealing a wellbore of reduced external size, and in particular apparatus of lesser lateral dimension and reduced weight when compared with a typical ram type BOP.
A further aim of the invention is to provide apparatus for sealing a wellbore, where the apparatus has a reduced requirement for hydraulic actuation when compared with a typical ram type BOP.
Further aims and objects of the invention will become apparent from the following description.